1. Field of the Invention
The present invention relates to a magnetoresistive device in which a large change in magnetoresistance is caused in a low magnetic field, and also relates to a magnetoresistive head which is configured using such a magnetoresistive device and is suitable for high-density magnetic recording and reproducing operations.
2. Description of the Related Art
A magnetoresistive sensor (hereinafter, simply referred to as an "MR sensor") and a magnetoresistive head (hereinafter, simply referred to as an "MR head") using a magnetoresistive device (hereinafter, simply referred to as an "MR device) have been under development. The term "a magnetoresistive device" indicates a device which varies an electric resistance depending on the magnetic field externally applied. The characteristic of the MR device is generally represented by a ratio of change of magnetoresistance (hereinafter abbreviated as an MR ratio). The MR ratio is defined by the following equation:
MR ratio (%)=(R(maximum)-R(minimum))/R(minimum).times.100
where R(maximum) and R(minimum) denote the maximum value and the minimum value of the resistance of the MR device when a magnetic field is applied to the MR device.
Conventionally, a permalloy made of Ni.sub.0.8 Fe.sub.0.2 or an alloy layer made of Ni.sub.0.8 Co.sub.0.2 is mainly used as the magnetic body. In the cases where such magnetoresistive materials are used, the MR ratio is about 2.5%. In order to develop an MR device having higher sensitivity, an MR device having a higher MR ratio is required. It was recently found that Fe/Cr! and Co/Ru! artificial multilayers in which antiferromagnetic coupling is attained via a metal non-magnetic thin layer made of a material such as Cr and Ru exhibit a giant magnetoresistance (GMR) effect in a ferromagnetic field (1 to 10 kilooersteds (kOe)) (Physical Review Letter Vol. 61, p. 2472, 1988; and Physical Review Letter Vol. 64, p. 2304, 1990). However, since these artificial multilayers require a magnetic field of several to several tens of kOe in order to attain a large MR change, such artificial multilayers cannot be practically applied to a magnetic head and the like.
In addition, it was also found that an Ni--Fe/Cu/Co! artificial multilayer using magnetic thin layers made of Ni--Fe and Co having different coercive forces in which they are separated by a metal non-magnetic thin layer made of Cu and which are not magnetically coupled exhibits a giant magnetoresistance effect, and an artificial multilayer which has an MR ratio of about 8% when a magnetic field of about 0.5 kOe is applied at room temperature has been hitherto obtained (see Journal of Physical Society of Japan, Vol. 59, p. 3061, 1990). However, in the case of using an MR material of such a type, a magnetic field of about 100 Oe is required for attaining a large MR change. Moreover, the MR asymmetrically varies from the negative magnetic field to the positive magnetic field, i.e., the MR exhibits poor linearity. Thus, such an artificial multilayer has characteristics which are not suitable for practical use.
Moreover, it was also found that Ni--Fe--Co/Cu/Co! and Ni--Fe--Co/Cu! artificial multilayers using magnetic thin layers made of Ni--Fe--Co and Co in which RKKY-type antiferromagnetic coupling is attained via Cu exhibit a giant magnetoresistance effect, and an artificial multilayer which has an MR ratio of about 15% when a magnetic field of about 0.5 kOe is applied at room temperature has been hitherto obtained (see Technical Report by THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS of Japan, MR91-9). However, in the case of using an MR material of such a type, the magnetoresistance varies substantially linearly from zero to the positive magnetic field and characteristics which are sufficiently suitable for the application to an MR sensor are obtained. Nevertheless, in order to obtain a large MR change, a magnetic field of about 50 Oe is also required. Thus, such a characteristic of the film is not appropriate for the application to an MR magnetic head which is required to be operated at most at 20 Oe and preferably less.
As a film which can be operated even when a very weak magnetic field is applied, a spin-valve type film in which Fe--Mn as an antiferromagnetic material is attached to Ni--Fe/Cu/Ni--Fe has been proposed (see Journal of Magnetism and Magnetic Materials 93, p. 101, 1991). The operating magnetic field of an MR material of this type is actually weak, and a good linearity is observed. However, the MR ratio thereof is as small as about 2%, and the Fe--Mn layer has a poor corrosion resistance and a low Neel temperature, so that the device characteristics disadvantageously exhibit great temperature dependence.
Furthermore, a spin-valve film having a structure of Ni--Fe/Cu/Co--Pt or the like including a hard magnetic material such as Co--Pt instead of an antiferromagnetic material has also been suggested. In such a case, a parallel alignment of the magnetizations and an antiparallel alignment of the magnetizations are caused by rotating the magnetization direction of a soft magnetic layer at a coercive force equal to or less than that of a hard magnetic layer. However, even when such a structure is employed, it is still difficult to improve the soft magnetic characteristics of a soft magnetic layer. Thus, this structure has not been used practically, either.
In conventional magnetoresistive devices utilizing a giant magnetoresistance effect, a crystalline Ni--Fe alloy or Ni--Fe--Co alloy is used for a soft magnetic layer. Thus, such magnetoresistive devices cannot totally eliminate magnetocrystalline anisotropy. Consequently, in such conventional magnetoresistive devices, the soft magnetic characteristics thereof are still poor and the operating magnetic field thereof cannot be strong.
On the other hand, it was recently reported that a magnetoresistance effect of about 5.4% is attainable even in a spin-valve film having a structure of Co--Fe--B/Cu/Co including a soft magnetic layer made of a Co--Fe--B amorphous alloy (see Japanese Journal of Applied Physics, Vol. 34, pp. L112-L114, 1995). Since an amorphous alloy layer used as a soft magnetic layer exhibits more satisfactory soft magnetic characteristics as compared with those of a conventional crystalline Ni--Fe alloy layer or Ni--Fe--Co alloy layer, a magnetoresistive device having higher magnetic field sensitivity can be fabricated. However, in the spin-valve film including the Co--Fe--B/Cu/Co structure, it was difficult to simultaneously fulfill the incompatible purposes of obtaining a sufficiently large MR ratio and a sufficiently high magnetic field sensitivity.